Two components integral with an electrically driven tool or device is the motor that converts electrical energy into mechanical energy for accomplishing a desired task and the complementary motor control circuit. One class of electrically driven tools that include both a motor and a motor controller is the electrically powered surgical tool. These tools typically include a handpiece in which the motor is housed. A complementary medical accessory such as a drill, a saw blade or a wire driver is coupled to the handpiece. A gear assembly transfers the rotational power of the motor rotor to the medical accessory in order to cause the accessory to engage in the desired motion. The development of powered surgical tools has lessened the physical strain of surgeons and other personnel when performing medical procedures on a patient. Moreover, most surgical procedures can be performed more quickly, and more accurately, with powered surgical tools than with the manual equivalents that proceeded them.
The motor control circuit regulates the application energization current to the motor in order to cause the motor rotor to turn both at the desired speed and in the desired direction. One such control circuit is disclosed in the Inventor's U.S. Pat. No. 5,747,953, entitled CORDLESS, BATTERY OPERATED SURGICAL TOOL, issued May 5, 1998, which is incorporated herein by reference. The foregoing patent discloses a motor control circuit response that receives as inputs signals indicating: the direction the motor is to be driven, forward, reverse or oscillatory; the speed at which the motor is be driven; and the speed the motor rotor is turning. Based on these input signals, this motor controller selectively ties the windings integral with the motor to a voltage source or ground. The tieing of the windings to the voltage source or ground causes appropriate electromagnetic fields to develop around the windings to, in turn, cause the motor rotor to turn in the desired direction at the desired speed. In the event the rotor is turning faster than the desired speed, the circuit disclosed in this patent is ties the windings to ground so that the windings develop a braking electromotive force that slows the rotor down. The circuit disclosed in this patent is also has failsafe features that prevent the windings being placed in the braking mode if such act could potentially be harmful to the motor or the tool in which it is installed.
The motor control circuit disclosed in the above patent has proven to a very useful circuit for providing precision control for a D.C. powered motors. It should be noted however, that this circuit has numerous discrete components. Recently, there has been interest in providing a motor control circuit in which as many of sub-circuits that comprise the circuit as possible are contained on a single integrated circuit chip. An advantage of so constructing a motor control circuit is that it decreases the size of the circuit and reduce its cost of manufacture and maintenance while, at the same time, increasing its reliability. However, the above circuit includes a large number of both analog and digital signal processing components. Accordingly, it has proven very difficult to fabricate a single integrated semiconductor component, a chip, in which a substantial number of the components that form this motor control circuit are present.